If your engine is using oil and emits a large puff of oil smoke when you snap open the throttle after a period of coasting, throttle closed (for example when running downhill) then it is likely drawing oil down the inlet valves and into the combustion chamber where it is burned,
The answer is to install valve stem oil seals as this controls the passage of oil between the valve stem and the valve guide. The cylinder heads need to be removed for this exercise and new valve guides fitted so this may be an action best undertaken during an engine overhaul.